An existing method of manufacturing a semiconductor device comprises forming a plurality of gate structures that are spaced apart from each other at a distance on a substrate and forming a gap-fill layer on and between the gate structures. When the gap-fill layer is formed on and between the gate structures, it is generally desired to reduce air gaps in the gap-fill layer. In order to reduce the air gaps, use of a gap-fill layer having a lower density and greater flowability is preferred. After forming the gap-fill layer, a contact hole is formed in the gap-fill layer. However, if the density of the gap-fill layer is low, the contact hole may have a bowing profile or a wide mouth profile. Therefore, an additional process is needed to increase the density of the gap-fill layer before forming the contact hole.